linux-kernel - Re: [RFC][PATCHv2 3/4] serial: introduce uart

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Date:   Wed, 12 Dec 2018 12:08:38 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Jiri Slaby <jslaby@...e.com>, linux-kernel@...r.kernel.org,
        Daniel Wang <wonderfly@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Peter Feiner <pfeiner@...gle.com>,
        linux-serial@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC][PATCHv2 3/4] serial: introduce uart_port locking helpers

On Sat, Dec 08, 2018 at 12:12:49PM +0900, Sergey Senozhatsky wrote:
> On (10/16/18 14:04), Sergey Senozhatsky wrote:
> [..]
> > - The first entry point is console ->write() callback, which we call
> >   from printk(). A possible deadlock scenario there is:
> > 
> >   CPU0
> > 	<NMI>
> > 	spin_lock_irqsave(&port->lock, flags)      << deadlock
> > 	serial_foo_write()
> > 	call_console_drivers()
> > 	console_unlock()
> > 	console_flush_on_panic()
> > 	panic()
> > 	<NMI/>
> > 	spin_lock_irqsave(&port->lock, flags)
> > 	serial_foo_write()
> > 	call_console_drivers()
> > 	console_unlock()
> > 	printk()
> > 	...
> 
> [..]
> > - The rest (of entry points) requires a bit different handling.
> >   Let's take a look at the following backtrace:
> > 
> >   	CPU0
> > 	<IRQ>
> > 	spin_lock_irqsave(&port->lock, flags)      << deadlock
> > 	serial_foo_write()
> > 	call_console_drivers()
> > 	console_unlock()
> > 	printk()
> > 	__queue_work()
> > 	tty_flip_buffer_push()
> > 	spin_lock_irqsave(&port->lock, flags)
> > 	serial_foo_handle_IRQ()
> > 	<IRQ/>
> >
> >   Serial drivers invoke tons of core kernel functions - WQ, MM, etc. All
> >   of which may printk() in various cases. So we can't really just
> >   "remove those printk-s". The simples way to address this seems to be
> >   PRINTK_SAFE_CONTEXT_MASK.
> 
> serial/UART and printk guys, sorry to bother you, do you hate this
> idea of removing console_driver->CORE KERNEL->printk->console_driver
> deadlock path? Or is there any chance we can move forward?

If done in a sane manner, no objection from me.